The employment of flotation to upgrade valuable minerals from pyrite and other gangue minerals is generally performed at an alkaline pH. Alkalinity is controlled by the addition of lime or other alkaline compounds. Lime is normally employed as it is a relatively inexpensive reagent; however, large amounts of lime and other reagents are required when the water available to the flotation circuit possesses a high buffering capacity. In other words, a large amount of lime is necessary to alter and maintain the pH at the optimal operating conditions. The addition of lime can also depress the flotation of minerals such as chalcopyrite, sphalerite, molybdenite, pyrite, pyrrhotite, and gold and other precious metals via the deposition of calcium on the metal surface.
Commonly, in sulfide flotation, the effectiveness of flotation agents is controlled by the level of alkalinity or acidity in the flotation feed or pulp pH regulators such as lime, soda ash and, to a lesser extent, caustic soda, are often employed as the pH controlling agents. Lime is the most commonly used agent because of its cost, availability and ability to maintain pH values of pH 10.5 and above. Adjustment of the pH of the pulp to pH 11.0 is required to depress the gangue sulfide minerals of iron, such as pyrite and pyrrhotite. The costs associated with adding lime can be significant and the effectiveness of lime as a depressant has been shown herein to be reduced in waters containing high levels of dissolved salts or are highly buffered.
Other sulfide depressants have been employed to depress pyrite, such as cyanide or sodium hydrosulfide, in conjunction with pH modification. They cannot be used over a wide pH range and require high pH values, so that high lime consumption remains an issue. In addition these depressants may not be sufficiently selective at economic dosages. The use of sulfoxy compounds to improve the recovery of sulfide minerals was described as far back as U.S. Pat. No. 2,154,092 to Hunt. This patent describes a process to treat ores containing carbonaceous or graphitic substances associated with gangue components. These carbonaceous substances may either remain with the valuable ore mineral during flotation and reduce the grade or coat the valuable minerals, thereby reducing their recovery by flotation. To prevent this, sulfur dioxide or any other reducing gas, is added to the pulp, without mixing it with air, to inhibit the flotation of the deleterious gangue and carbon coated minerals.
When the sulfur dioxide gas is added, Hunt states that the resulting pH of the pulp water is usually on the acid side (<pH 7). In some cases, depending on the natural alkalinity of both the ore and the milling water, the pulp may remain alkaline. The process can be carried out when the pulp is either acid or alkaline.
Hunt teaches that the reducing gas may also be internally generated in the ore pulp itself by the action of one or more suitable chemicals. For example, when sulfuric acid and an alkaline (base) or alkaline earth sulfite, bisulfite, or thiosulfate are added to an ore pulp, sulfur dioxide will be one of the products resulting from the interaction.
A number of other patents have employed sulfoxy compounds in sulfide flotation circuits.
U.S. Pat. No. 5,171,428 to Beattie, et al., describes a process to separate arsenopyrite from a mixture with pyrite by contacting the mixture with a sulfitic agent providing HSO3− ion. The process is performed at an elevated temperature and a pH below about pH 8 for a period sufficient to impart a selective depression of arsenopyrite.
U.S. Pat. Nos. 6,032,805, and 6,092,666 to Clark, et al., disclose a method for reducing the consumption of alkaline pH modifiers by using a sulfoxy radical-containing reagent. Prior to or simultaneously with the introduction of the sulfoxy radical-containing reagent, a non-oxidizing gas (such as an inert or reducing gas) is added in a quantity sufficient to achieve a chemical environment conducive to the flotation separation of minerals. Prior to collector and frother addition but after contact with the non-oxidizing gas, the slurry, only when necessary, is aerated by an oxidizing gas to a particular dissolved oxygen concentration or electrochemical potential suitable for flotation.
U.S. Pat. No. 6,041,941 to Newell, et al., presents a similar process to Clark, et al., with the aim of reducing reagent consumption and mineral scale formation in flotation circuits. In the process of Clark, et al., the non-oxidizing gas is added to prevent the oxidation of the sulfoxy radical. The non-oxidizing gas is introduced during the reagent conditioning and flotation stages. At these stages, the dissolved oxygen in the slurry is most likely to degrade the sulfoxy compounds and result in scale formation.
There is a need for a process that can separate valuable metal-containing sulfide minerals from other sulfide minerals, particularly sulfidic gangue minerals, while controlling levels of reagent consumption in waters with a significant range of buffering capacities and/or salinities, without the addition of lime or other pH modifiers.